1. Field of the Invention
This invention relates to a method for quantitatively analyzing digital images of approximately elliptical body organs, and in particular, two-dimensional echocardiographic images.
2. Related Art
Two-dimensional ultrasonic imaging is used as an important non-invasive technique in the comprehensive characterization of a number of body organs. In ultrasonic imaging, a sound pulse is sent along a ray from a transducer towards the organ that is being imaged. The pulse is attenuated and reflected when it hits a medium with an acoustic impedance different from that of the medium in which the pulse is traveling. The time the sound pulse takes in transit is a measure of the distance of the boundary from the transducer, and the amount of energy that is reflected is a measure of the difference of the acoustic impedance across the boundary. (In practice, since the energy of the pulse diminishes as it travels, post-processing Of the reflected signal includes time gain control that compensates the attenuation of the signal over time). Assuming the pulse travels at a single speed in the body, and by taking different rays across the plane, a two-dimensional record of the received energy in spatial coordinates represents a cross-sectional view of the imaged organ.
Echocardiography is the application of ultrasonic imaging to the heart. Echocardiography has experienced widespread acceptance in the evaluation of cardiac disease and in characterizing the structure and function of the heart. This acceptance is in large part due to its non-invasive nature, and its real-time capability for observing both cardiac structure and motion. Using echocardiography, a considerable amount of quantitative information can be obtained concerning cardiac anatomy, chamber diameter and volume, wall thickness, valvular structure, and ejection fraction.
The real-time capability of echocardiography can be used to measure the variation of the shape of head structures throughout the cardiac cycle. These analyses require the complete determination of inner (endocardial) and outer (epicardial) boundaries of the head wall, particularly of the left ventricle. Present evidence indicates that sensitive detection of ischemic disease with two-dimensional echocardiography requires knowledge of the endocardial border on echocardiographic frames throughout the cardiac cycle as well as at end-diastole and end-systole.
Since both global and regional left ventricular function are major variables used to determine prognosis in cardiac disease, there is considerable interest in the ability to quantitate function indexes from echocardiographic images. Presently, such indexes (e.g., left ventricular chamber volume and left ventricular ejection fraction) are calculated from observer-defined cardiac boundaries traced with either a light pen or a digitizing tablet. Tracing of endocardial borders on two-dimensional echocardiograms is tedious and the borders are highly subjective. Indeed, in most systematic studies, substantial intraobserver and interobserver variability has been found in such observer-defined cardiac boundaries.
Manually defining such boundaries becomes increasingly labor intensive when the analysis of a complete cardiac cycle is needed to provide a description of the systolic and diastolic wall motion pattern, or when a number of echocardiographic frames have to be processed in order to obtain a long period time-history of cardiac function. It is therefore desirable to automate as much as possible the determination of boundaries of echocardiographic images. Automated definition of the boundaries would improve the reliability of the quantitative analysis by eliminating the subjectivity of manual tracing.
Finding boundaries in echocardiograms automatically by computers is often difficult because of the poor quality of the echocardiographic images. The lack of clear definition of the boundaries is due to the intrinsic limitations of echo imaging, such as low image intensity contrast, signal dropouts in the image, and boundary discontinuity in any given frame. ("Dropouts" occur where sound waves are reflected from two different levels in a structure and the reflected waves arrive simultaneously at the face of the transducer but out of phase, causing a cancellation of their amplitudes. Thus, no return signal is perceived at that depth).
The poor quality of echocardiograms is also attributable to the reverberations of the initial sound pulse, and "speckle" noise, caused by the back scattering of the incident wave front after it hits the tissue microstructures (this phenomenon produces a very fine texture, a "salt and pepper" pattern, that is superimposed on the image). Another limitation of echocardiographic imaging is that Sound reflection is not very pronounced when the angle between a boundary of the heart and the ray along which the sound pulse is traveling is small. Hence, the lateral wall boundaries of the heart are usually not very well defined in echocardiographic images. Thus, in imaging the left ventricle, typically only the anterior and posterior cardiac walls are well-defined.
In the past several years, advances in computer data processing technology have allowed the application of several different automatic boundary detection methods to echocardiographic images. However, most researchers have had difficulties with image enhancement and boundary detection with echocardiographic images because of the low signal-to-noise ratio and large discontinuities in such images. Thus, automated border detection has been reported in two-dimensional echocardiographic images, but only when the images are of good quality and certain smoothing techniques are employed prior to edge detection in order to render the endocardial edge more continuous. An overview of the field is set forth in Chapter 22 of Echocardiography in Coronary Artery Disease, Kerber, Richard E., Ed., entitled Applications of Automatic Edge Detection and Image Enhancement Techniques to Two-Dimensional Echocardiography and Coronary Disease, by E. A. Geiser (Futura Publishing Company, Mount Kisco, N.Y. 1988 ISBN 087993-325-9).
Consequently, there is a need for a method for automatically determining quantitative characteristics of ultrasonic images, especially echocardiographic images. In particular, there is a need for a method that can automatically determine the center of an imaged structure and approximate the borders of such a structure. With respect to echocardiographic images, there is a need for an automated system that can determine the canter of the left ventricle, approximate both the endocardial and epicardial borders, and estimate cardiac wall motion without the necessity of any user input. In addition, it is also desirable to automatically detect the presence of a flattened interventricular septum caused by pressure or volume overload from the right ventricle.
The present invention provides such a method. The invention uses mathematical techniques implemented in computer software which allows near real-time automatic quantitation of cardiac wall motion, cardiac wall thickness, and the area change fraction of two-dimensional short-axis echocardiographic image studies. Some applications of this system would be in a hospital at a patient's bedside, or in an echocardiography suite, where a detailed evaluation of a patient's cardiac health is required. Another application would be in an operating room, where elderly patients with significant coronary arterial disease are to undergo surgery. The invention provides a means for continuously and automatically monitoring a patient's heart for possible ischemic changes during surgery. Thus, an attending physician could be warned of a potential danger without requiring continuous physician monitoring of a patient, or invasive catheter placement.